Impedance spectroscopy on porous materials:: A general model and application to graphite electrodes of lithium-ion batteries

Electrochemical impeclance spectroscopy (EIS) was applied to porous negative graphite electrodes for lithium-ion batteries ill the EC:DMC, 1M LiPF6 electrolyte. The effect of porosity oil the electrode response time was studied and a theoretical model was developed, based on free path of the current lines between subsequent reaction sites. The effect of porosity oil the electrode response is evidenced by the impedance spectra ill which the high frequency Capacitive semicircle is distorted. Fresh electrodes (before the formation of the solid electrolyte interphase, SEI) and cycled electrodes have different shapes of the impeclance spectra indicating a change of processes at the surface. In particular, the shape of the spectrum for a fresh electrode call he related to all adsorption process. Impedance spectra of fresh electrodes were fitted using a simple model that considers porosity and the assumed electrochemical processes, giving good agreement between model and data. A correlation was found between adsorption sites and irreversible chat-e capacity ill the first cycle. (c) 2008 Elsevier Ltd. All rights reserved.